An activation controller has been proposed that activates an occupant protection device such as an airbag to protect an occupant from a collision. Such an activation controller is configured in a redundant manner to ensure a fail-safe operation.
For example, as shown in FIG. 7A, a conventional activation controller includes two sensors, one of which is a main sensor and the other of which is a safing sensor (i.e., arming sensor). Each of the main sensor and the safing sensor is an acceleration sensor and detects deceleration (i.e., a negative acceleration) of a vehicle. The main sensor is connected to a main central processing unit (CPU). The main CPU determines whether a vehicle collision occurs based on an output signal of the main sensor. The safing sensor is connected to a sub CPU. The sub CPU determines whether a safing condition is met based on an output signal of the safing sensor. The activation controller allows an occupant protection device to be activated, only when the sub CPU determines that the safing condition is met. Thus, the activation controller is configured in a redundant manner to prevent false activation of the occupant protection device. However, a CPU is expensive, and the activation controller requires two CPUs. Therefore, the activation controller is costly.
As disclosed, for example, in JP-A-H9-222437 and US 20040204810 corresponding to JP-A-2004-306920, an activation controller has been proposed that uses one CPU and is configured in a redundant manner. In the activation controller, as shown in FIG. 7B, a main sensor is connected to a CPU, and a safing sensor is connected to a simple integrated circuit (IC) having a comparator. The CPU determines whether a vehicle collision occurs based on an output signal of the main sensor. The comparator determines whether a safing condition is met by comparing an output signal of the safing sensor with a predetermined reference value. Specifically, the comparator determines that the safing condition is met, when the output signal of the safing sensor exceeds the reference value. A trigger signal for allowing the occupant protection device to be activated is outputted from the comparator, only when the safing condition is met. Thus, the activation controller prevents false activation of the occupant protection device by using one CPU.
The IC having the comparator is cheaper than a CPU due to its simple configuration. However, due to the simple configuration, the IC has very limited function. Therefore, although the safing condition needs to be customized according to vehicles, the IC cannot achieve the customization of the safing condition.